Low-molecular weight branched siloxanes include branched tetrasiloxanes such as methyltris(trimethylsiloxy)silane, 3-acryloxypropyltris(trimethylsiloxy)silane and 3-methacryloxypropyltris(trimethylsiloxy)silane, and branched pentasiloxanes such as tetrakis(trimethylsiloxy)silane. They are used as industrial oils, cosmetic oils, detergents, reactants for electronic materials, monomers for contact lenses and the like. Besides, 5-norbornenyltris(trimethylsiloxy)silane, 2-(5-norbornenyl)ethyltris(trimethylsiloxy)silane, and 2-(5-norbornenyl)ethyltris(dimethylvinylsiloxy)silane are important as monomers for ring-opening metathesis polymerization. In particular, 2-(5-norbornenyl)ethyltris(dimethylvinylsiloxy)silane is also useful as a crosslinker and a reactant for the synthesis of dendrimers. Of these applications, the use as cosmetic oils requires a high purity. It is desired to suppress the level of impurity components of one silicon atom to 0.1 wt % or below because they can be irritative to the skin, and to minimize the amount of linear and cyclic compounds of two or three silicon atoms because they can also be irritative to the skin. Similarly a high purity is desired for 3-acryloxypropyltris(trimethylsiloxy)silane, 3-methacryloxypropyltris(trimethylsiloxy)silane and p-styryltris(trimethylsiloxy)silane since they are used as monomers for contact lenses.
It is desired that methyltris(trimethylsiloxy)silane for use as cosmetic oils and 3-acryloxypropyltris(trimethylsiloxy)silane, 3-methacryloxypropyltris(trimethylsiloxy)silane and p-styryltris(trimethylsiloxy)silane for use as contact lens-forming monomers contain reduced levels of low-boiling silicon-containing components and be of high purity.
Many methods are known for the preparation of branched siloxanes. Known methods for preparing methyltris(trimethylsiloxy)silane, for example, include the following.    (1) Co-hydrolysis of methyltrichlorosilane and trimethylchlorosilane in the presence of methanol is described, for example, in WO 2001/15658 and JP-A 2002-68930.    (2) Reaction of methyltrichlorosilane with hexamethyldisiloxane in the presence of perchloric acid catalyst is described, for example, in Dokl. Akad. Nauk, SSSR, 1976, 227, 362-365.    (3) Reaction of methyltriethoxysilane with hexamethyldisiloxane in the presence of an acidic ion-exchange resin is described, for example, in J. Organomet. Chem., 1988, 340, 31-36.    (4) Reaction of methyltrialkoxysilane with hexamethyldisiloxane in the presence of a carboxylic acid and an acid catalyst is described, for example, in JP-A 11-217389.    (5) A method of adding conc. sulfuric acid to methyltrimethoxysilane, hexamethyldisiloxane and methanol and then adding dropwise a mixture of water and methanol thereto is described, for example, in WO 2001/15658 and JP-A 2002-68930.
These methods, however, have problems. Method (1) needs a large volume of water and results in very low yields due to low selectivity of reaction. Method (2) is unsuitable as an industrial process because perchloric acid which is difficult to handle must be used as the catalyst. Method (3) must use hexamethyldisiloxane in large excess in order to promote conversion, but a large amount of a reaction intermediate, 1,1,1,3,5,5,5-heptamethyl-3-ethoxytrisiloxane is left behind. Thus method (3) is not suitable for producing methyltris(trimethylsiloxy)silane with a high purity. In method (4), a by-product having a similar boiling point, 1,1,1,3,5,5,5-heptamethyl-3-methoxytrisiloxane is produced along with the end compound, methyltris(trimethylsiloxy)silane. The yield is 82% at the stage of reaction solution. However, after the reaction solution is distilled for purification to isolate methyltris(trimethylsiloxy)silane with a high purity, the yield as isolated is estimated to be significantly lower. Method (5) produces methyltris(trimethylsiloxy)silane with a high purity, but in a yield of 70%, which is unsatisfactory. The products of methods (3) to (5), which contain monomethoxysilanes and monohydroxysilanes such as 1,1,1,3,5,5,5-heptamethyl-3-methoxytrisiloxane and 1,1,1,3,5,5,5-heptamethyl-3-hydroxytrisiloxane as impurities, must be distilled or otherwise processed to remove the impurities before methyltris(trimethylsiloxy)silane can be obtained in high purity.
A method of converting methyltris(trimethylsiloxy)silane containing monomethoxysilanes and monohydroxysilanes such as 1,1,1,3,5,5,5-heptamethyl-3-methoxytrisiloxane and 1,1,1,3,5,5,5-heptamethyl-3-hydroxytrisiloxane as impurities to a high purity form by way of reaction to reduce the content of monomethoxysilanes and monohydroxysilanes is unknown in the art.